We propose the study of renal function by micropuncture and X-ray spectrometry through electron probe excitation (EPA). The role of PTH and Calcitonin will be investigated by micropuncture in TPTX rats, in isolated perfused kidneys, in collecting ducts of hamsters and in perfused proximal tubule rabbits. The effect of drug on potassium transport in the proximal tubule and chlorine transport in the distal tubule will be investigated after sulfate loading. The role of the pelvic urine in the concentrating mechanism will be investigated by electron probe analysis of kidney kept frozen during the X-ray spectrometric analysis.